Deep fat frying has become one of the most popular methods of cooking in domestic, restaurant and industrial establishments throughout the World. Because of the high temperatures involved (typically 160 to 200° C.) it is relatively quick, cooks food right through to the middle, generates a distinctive crust on the food and perhaps most importantly produces rich and complex flavours and food textures, which are very appealing to the consumer.
Frying, whether carried out in oils or fats, however also has a number of well-known disadvantages.
Cooking oil is expensive: high end olive oils are more expensive per liter than petrol or diesel and the price of even lower end cooking oils is comparable to that of petrol or diesel. Cooking oils have to be replaced frequently as the oils degrade during the cooking process, as more fully explained hereinafter. Also cooking oils (and their breakdown products) are absorbed by the food cooked in them which therefore necessitates the operator of a fryer to regularly keep the oil or fat topped up by the addition of extra cooking oil or fat. Cooking in oil therefore comes at a relatively high price compared to boiling in water or roasting in air.
The frequent changing of cooking oil in kitchens, restaurants and industrial sites where food items are manufactured is also a labour intensive and laborious task, which is costly and increases equipment down-time.
Unfortunately it is not possible to extend the life of cooking oils and fats merely by filtering out food debris particles, which frequently accumulate within them. During use cooking oils and fats do not remain unaltered but begin to chemically breakdown. Cooking oils and fats are commonly referred to as triglycerides but are in fact triacylglycerols: i.e. triesters of glycerol (1, 2, 3 propanetriol, which is commonly referred to as glycerine) and three fatty acids. The fatty acids do not need to be of the same type and frequently are not. Common chain lengths for the fatty acids, as determined by gas liquid chromatography, are 12 to 24 carbon atoms with 16 and 18 being particularly favoured. The breakdown of such triglycerides is complex, dependent on numerous factors and is subject to numerous feedback effects but involves three well-understood basic mechanisms: oxidation, polymerisation and hydrolysis.
Oxidation
Oxidation occurs when air comes in contact with frying oil, (see for example Josephson and Lindsey 1987, Journal of Food Sciences, 52, 328 and Fischer and Muller 1991, Potato Research, 34, 159). Oxygen from the air reacts with the two unsaturated carbons at the double-bond via a free radical initiated reaction. The oxidation reaction is promoted by high cooking temperatures (typically 190° C. and above), the presence of metals (including in particular copper and iron) and the presentation to the air of a large surface area of the oil as well as exposure to UV light, which promotes free radical formation. Initially hydroperoxides are produced but these are unstable and at frying temperatures they rapidly break down (via e.g. fission, dehydration and free radical formation) to produce a wide array of secondary oxidation products including polymers, acids, alcohols, esters, aldehydes, methyl ketones, lactones, alcanes, aromatics and other hydrocarbons, (see Belitz and Grosch 1999, Food Chemistry, 2nd edition, Springer-Verlag, Berlin, p. 211).
Some of these secondary oxidation products are volatile and give rise to both pleasant rich flavours but some are also associated with rancid and offensive flavours. For example only 0.08 ppm of pentane is sufficient to reliably produce rancidity, (Warner et al. (1974) Journal of Food Science, 39, 761). Non-volatile compounds, such as core aldehydes, remain in the oil and are absorbed by the food.
Polymerisation
When cooking oil breaks down, the resulting products form both volatile low boiling point and higher boiling point non-volatile compounds. The non-volatile higher boiling point compounds remain within the frying oil and readily polymerize at frying temperatures above 190° C. or in isolated hot spots within the fryer. Such polymerisation products can then bond together to form larger clusters, which can accumulate as an insoluble layer on the surface of the oil, thus preventing water vapour, evaporating from food cooking in the oil, escaping from the oil's surface and thereby producing dangerous foaming, which can lead to fires and personal injury of kitchen staff.
The presence of the impermeable polymer layer in turn promotes more hydrolysis in what can become a runway feed-back driven process. Polymerisation also leads to an increase in the viscosity of the oil which reduces its ability to effect heat transfer and promotes yet more polymerisation. The increase in viscosity also increases the amount of energy required to effect cooking and thus increases energy bills.
Hydrolysis
Hydrolysis is caused by the reaction of water (a weak nucleophile) with the ester linkage in the triacylglycerol molecule to produce initially a diaglyceride and a free fatty acid, which then further breakdown to produce various compounds including lactones or simply boil off, depending on chain length, saturation and other factors. The presence of free fatty acids is frequently associated with a characteristic rancid or acidic flavour.
The production of free fatty acids in cooking oils is additionally problematical for several reasons.
Firstly free fatty acids are one of the main constituents of smoke haze and are both a fire and a health hazard. The smoke point of an oil is the temperature, at which it is seen to start smoking under specified test conditions. The flashpoint of an oil is the temperature at which volatile products are produced in sufficient concentration and quantity to allow ignition. The fire point of an oil is the temperature at which the rate of production of volatile products is sufficiently high to support continuous combustion of the gases emerging from the surface of the oil.
High levels of free fatty acid in cooking oils are associated with reduced smoke, flash and fire points and are thus a significant fire hazard. For example Weiss (Food Oils and Their Uses, Wesport, The AVI Publishing Co. 1983) found that a free fatty acid composition of 0.04% was associated with a smoke point of 218° C., a flashpoint of 327° C. and a fire point of 366° C. whereas for the same oil increasing the free fatty acid content to just 1% percent lead to the smoke point decreasing to 160° C., the flashpoint decreasing to 307° C. and the fire point dropping to 360° C.
In addition to being a fire hazard, an increase in the concentration of free fatty acids (and their break down products) in cooking oils also has deleterious effects on the preparation of food cooked in such oils.
Fatty acids and some of their breakdown products, having both distinct hydrophobic and hydrophilic regions, act as effective surfactants. The effect of the concentration of surfactants in cooking oil on the properties of the food cooked in such oil is well-known (see e.g. Blumenthal M M. A new look at the chemistry and physics of deep fat frying: food technology, 1991, 45:2, 68-71, 94). When for example chips are cooked in fresh unused cooking oil they are light in colour and do not have the rich complex aromas associated with fried potatoes. The oil, during this “break in” phase has only low levels of surfactants (such as free fatty acids), which means that the oil has a relatively high surface tension which prevents the oil having close contact with the food. The heat from the oil is not effectively transferred across the oil/wet-food barrier and the food is in part boiled rather than fried as the steam emerging from the food pushes a substantial amount of the oil away from its surface. As the oil is used further the amount of free fatty acid and other surfactants increases resulting in improved food quality. During the so-called optimum phase chips cooked in the oil are golden brown in colour and have a significant crust but with relatively low levels of oil being absorbed by the food, which is cooked through to the centre. For example fresh French fries will typically consist of about 10% by weight of oil, when cooking during the so called optimum phase. However as the oil is subject to both further hydrolysis and oxidation, the increase in free fatty acids and other surfactants decreases the surface tension significantly and ensures that the oil can rapidly bridge the otherwise immiscible oil food barrier. This results in the surface of, for example, chips having a characteristic dark and spotted appearance. Excessive contact with the oil rapidly dries the surface region of the food thus trapping moisture deeper in the food and inhibiting heat penetration deeper within the food's centre, which therefore typically is undercooked. The resulting greasy chip with an oil content by weight of typically in excess of about 20%, with a dark spotted exterior and undercooked centre, is familiar to many who have eaten at down market fast food establishments, which do not change their cooking oil often enough.
The absorption of excessive amounts of cooking oils by food cooked in the oil also very significantly increases the calorific value of the food, thus giving many consumers extra calories they do not need and promoting obesity and the numerous health problems associated with it including in particular type II diabetes.
Further the absorption of excessive amounts of cooking oil by food has other important consequences for health. Hydrogenated vegetable oils and fats are widely used in cooking due mainly to their increased stability, resistance to oxidation, longer shelf-life and their greatly increased resistance to rancidity.
However such oils contain increased amounts of trans-fatty acid side chains on the glycerol backbone, which are a material health hazard. After ingestion most of the initial digestion of cooking oils is accomplished in the stomach via specialist pancreatic enzymes (lipases) and bile secretions. The resultant fatty acids and glycerol are then absorbed by cells lining the intestines called enterocytes, where they are re-esterified into triglycerides and transported to the liver as chylomicrons. When the chylomicrons reach the liver, the fatty acids are repackaged into triacylglycerols and phosphatidylcholine and thence into lipoproteins.
High levels of trans fatty acids in the diet are associated with raised serum levels of low density lipoprotein (LDL) cholesterol and with lower levels of high density lipoprotein (HDL) cholesterol in humans. Raised serum LDL and reduced serum HDL levels are associated with coronary artery disease, increased risk of stroke and elevated blood pressure as they decrease the health of the endothelium, the cells lining the arteries of the body which are essential for good cardiovascular health. Studies in humans further demonstrate that trans fats increase inflammation in the body, a potent risk factor for cardiovascular disease, diabetes, and other diseases. Studies in primates have demonstrated that trans fats cause weight gain, especially increasing abdominal fat, which has the greatest metabolic consequences, and is associated with insulin resistance, a known precursor to type II diabetes.
For all these reasons the amount of trans-fatty acids absorbed in the diet should be kept at low levels. One way of achieving that is to reduce the amount of hydrogenated cooking oil absorbed by fried food.
Various ways have been suggested to prolong the useful life of cooking oils. Some of these involve the step of removing the cooking oil from the fryer, followed by the step of subjecting it to one or more treatment methods to remove the contaminants before finally returning the treated oil back to the fryer. Other methods provide for at least the complete cessation of the cooking process, treatment and then the recommencement of the use of the oil.
Oil Removal and Treatment Methods
U.S. Pat. No. 4,112,129 (Duensing et al., Johns Manville) discloses a method of filtering the cooking oil through a composition comprising by weight (i) 47 to 59 parts diatomite, (ii) 28 to 36 parts synthetic calcium silicate hydrate and (iii) 12 to 24 parts synthetic magnesium silicate hydrate.
U.S. Pat. No. 4,681,768A (Mulflur W Jospeph et al) discloses a method for the continuous treatment of cooking oil with a filter made from synthetic calcium silicate. The method involves removal of the oil from the fryer, passing it through the filter and then passing it back into the fryer.
GB 2006729 (Johns Manville) discloses a method for filtering used cooking oils to remove free fatty acids, which uses synthetic calcium silicate but does not disclose an in situ solution suitable for unadapted fryers.
U.S. Pat. No. 5,870,945 discloses a filter cartridge for fitting to a fryer, which includes a mesh housing for containing filtering material which is used to treat the cooking oil outside the fryer prior to its return to the fryer.
U.S. Pat. No. 4,112,129A discloses a method for extending the life of cooking oil by removing free fatty acids which involves treating the oil with a composition of synthetic calcium silicate hydrate and synthetic magnesium silicate hydrate. U.S. Pat. No. 4,112,129A states that the method can be used with conventional cooking oil treatment systems but does not disclose an in situ solution suitable for unadapted fryers which do not have a treatment system.
EP 0226413A discloses a filter container provided with a removable filter bag but which cannot be used during the cooking operation.
U.S. Pat. No. 6,210,732 discloses a method of extending the life of cooking oil by the use of a blend of finely milled citric acid and calcium silicate powder, which is added to the hot oil, left for a certain length of time and then removed by treatment. The U.S. Pat. No. 6,210,732 invention cannot be used during the cooking process.
WO 91/11914A discloses a still further treatment method for used cooking oils, which uses an amorphous silica and alumina composition, which is either added to the hot oil and then filtered out or put in a container which is permeable to the oil but not the treatment composition. The invention disclosed cannot be used during the cooking operation.
U.S. Pat. No. 4,330,564A discloses a method of treating used cooking oil with a composition including a porous carrier, water and a food compatible acid, with the resultant residue being removed by treatment. The invention disclosed cannot be used during the cooking operation.
U.S. Pat. No. 3,947,602A discloses a method of treating cooking oil with a food compatible acid and a suitable adsorbent such as activated carbon. The invention disclosed cannot be used during the cooking operation.
U.S. Pat. No. 5,391,385A discloses the treatment of cooking oil with a mixture of 60-80% amorphous silica and 20 to 40% alumina, the mixture being placed in a permeable container which is then placed in the oil, the container being permeable to the oil but not to the mixture so that the adsorbent is not released into the oil and no treatment is required.
All the above treatment methods either require removal of the oil from the fryer and its treatment before reuse and/or cannot be carried out during the normal frying operation with standard frying equipment, which does not include in-line treatment equipment and a pump.
In Situ Treatment of Cooking Oil
Other methods are known for the treatment of cooking oil in the vessel where cooking takes place.
U.S. Pat. No. 4,764,384A discloses a method of treating used cooking oil with filtering media comprising synthetic amorphous silica, synthetic amorphous magnesium silicate and diatomaceous earth.
U.S. Pat. No. 5,354,570A discloses a method of frying food in cooking oils with a porous rhyolitic powder which selectively reduces the concentration of certain surfactants, whilst the cooking process is on-going.
JP 07-148073A discloses a method of treating cooking oil using finely pulverized zeolite stones which are inserted into a permeable bag which is itself placed into the fryer, with or without food also being present.
The above methods either require the addition of powders to the oil, which is undesirable as they may contaminate and change the texture and taste of any food cooked therein or require a further container to be added to the oil, which will often be problematical during use of the fryer due to space and other constraints.
The WO 2008/015481 and WO 2009/019512 Inventions
WO 2008/015481 and WO 2009/019512 (“the BBM Patents”) (BBM Technology Limited) disclose the use of cementious hydraulically set filters made from ordinary Portland cement (OPC), white cement clinker and mixtures thereof, in the form of standalone briquettes, blocks, pellets, granules or balls, which do not substantially leach calcium or magnesium into cooking oils.
The BBM Patents disclose the use of such filters in cooking oils (a) in situ actually in the frying chamber where the food is being fried during the frying operation and also (b) prior to first use when the cooking oil is in a storage container. WO 2009/019512 additionally discloses the use of film or sheet packaging that resists the ingress of water or water vapour for wrapping the filters, after they have been dried to remove free water after hydraulic setting.
The use of such filters in a typical restaurant, fast food outlet or pub kitchen fryer has the advantage that it does not require existing frying equipment of the type typically found in such establishments to be modified, the filters just being put into the oil. No filtration systems or pumps are required for use with such fryers. Such filters can be used in situ in the oil during use and can prolong the life of the cooking oil so treated typically by up to 100 percent by removing free fatty acids, certain aldehydes and other polar compounds. They also reduce the amount of cooking oil absorbed by the food and thereby the amount of trans fatty acids digested, where the cooking oil contains trans fatty acid side chains, as many hydrogenated vegetable oils do).
However the use of the filters disclosed in the BBM Patents is associated with foaming in use. Foaming is not the same as mere bubbling, which is caused largely by evaporating water from the food. During foaming, water vapour produced during the food cooking process is trapped just below an impermeable layer at the top of the oil and cannot escape so that, as the volume of trapped water vapour increases, the oily impermeable layer of bubbles rises up the sides of the fryer and can bubble over, causing either injury, fire or damage to property as well as an unwelcome interruption to busy kitchen staff.
Without wishing to be bound by any particular theory, it is thought that foaming is caused by free fatty acids, produced during hydrolytic breakdown of the oil, reacting with calcium ions that originate from the filter so as to produce calcium fatty acid salts—i.e. soaps. Even very small amounts of calcium can cause foaming as the charged calcium ions are pushed to the surface of the oil, allowing their fatty acid tails to come into close contact and either polymerise with each other or other oil breakdown products or produce a weakly van der waal bonded monolayer. We have found that when the overall concentration of calcium in the body of the oil is below 2 ppm then the calcium concentration in the foam layer can be as high as 57 ppm, evidencing the concentration of calcium in the foam layer.
Foaming is encountered with the filters the subject of the BBM Patents particularly in cooking environments where a lot of hydrolysis takes place due to large amounts of wet food being cooked, for example frozen pre-cooked chips. Wet foods cause more hydrolysis which produces more free fatty acids. Foaming is not encountered on every single use of such filters but any occurrence of it is likely to lead to the filters being permanently rejected for use by the end user. Foaming is perceived as being potentially dangerous and also it often occurs during busy times in the restaurants, fast food outlets and pubs where such fryers are located with the result that those who have ordered food in such establishments frequently have unacceptable waits for their food as the foaming oil is drained from the fryer and replaced by fresh cool oil, which then has to be heated to the frying temperature.
In our previous patent application (GB 1322146.0) we disclosed a method for reducing foaming by at least partially enclosing or surrounding the treatment media with a foraminous barrier. In trials, treatment elements enclosed in such a foraminous box were put below the heating element of electric fryers and greatly reduced the instances of foaming as can be seen from the table below:—
TABLE 1ExperimentPercentage of trialsNo. in GBNumber of instancesduring which foaming1322146.0of foamingwas observed13 out of 5 trials60%20 out of 5 trials 0%
TABLE 2no enclosures used% of% ofNon-establishmentsestablishmentsEnergyFoamingfoamingwith thiswith thissourceestablish-establish-energy sourceenergy sourcefor fryermentsmentswhich don't foamwhich foamElectricity 81260%40%Gas14 630%70%Total2218
Table 2 shows the results obtained from more extensive trials carried out in an 18 L electric fryer and a 20 L gas fryer, with two filters put in each fryer but with no enclosure being used. The filters were placed either under the heating element or on a flat surface of the gas fryer near the well. All frying was carried out in KTC vegetable oil, which contains E900—PDMS anti-foaming agent. Frying was carried out on five successive days using 10 lots of chips (900 gms each) so that after all trials had been completed 45 kg of chips had been cooked in the fryers.
It will be noted that on 40% of the frying twenty frying trials with the electric fryers that foamed occurred. Foaming was on average seen on the fourth day of frying with the average occurrence being during the 36th lot of frying on the fourth day, (SD+/−4.2 runs). It will be noted that 70% of the trials with the gas fryer foamed on average after the 23nd run with a SD+/−6.1 runs. With the gas fryer there were a lot more occurrences of foaming on the second and third day of frying. The effect of using an enclosure on foaming is tabulated below in table 3.
TABLE 3enclosure used% of% ofestablishmentsestablishmentsEnergyNon-with thiswith thissourceFoamingfoamingenergy sourceenergy sourcefor fryertrialstrialswhich don't foamwhich foamElectricity 23995% 5%Gas18 418%82%Total2043
It can be seen that the use of the foraminous enclosure reduced the incidence of foaming in electric fryers from 40% to less than 5% (2 occurrences in 41 frying runs). However it actually increased the incidence of foaming in gas fryers from 70% to 82%.
In our previous patent application (GB 1322146.0), we further disclosed a method for reducing foaming by the incorporation of microsilica into the mix, which due to its strong pozzolanic activity, accelerates the hydration reactions of the clinker phases (particularly alite). The fine microsilica particles fill the spaces between clinker grains thereby producing a denser paste and stronger overall cement and further reduces the amount of free calcium hydroxide produced by the hydration of alite and belite and promotes the production of calcium silicate hydrate gel. This reduction in free calcium hydroxide reduced the instance of foaming from 60% (3 out of 5 frying trials in example 1 of GB 1322146.0) to 20% (1 out 5 frying trials in example 4 of GB 1322146.0). However the use of microsilica did not completely eradicate the instances of foaming.
Thus, notwithstanding the invention disclosed in GB 1322146.0 there remains a pressing need for a method of reducing foaming still further in both electric fryers and gas fryers in particular.